This disclosure relates to an improved spectrophotometer color measurement calibration system, particularly for an on-line color measurement system with a spectrophotometer in the output path of a color printer for measuring the colors on printed test sheets, wherein a fully automatic re-calibration system is provided for the spectrophotometer, at little or no additional cost or modification of the color printer, and without requiring any manual operations or operator involvement.
This automatic re-calibration system assists in the effectiveness of such an on-line color measurement system in which a spectrophotometer may be mounted in the paper path of the moving copy sheets in the printer, preferably the output path, without having to otherwise modify the printer, or interfere with or interrupt normal printing, or the movement of the printed sheets in said paper path, and yet provide accurate color measurements of test color patches printed on the moving sheets as they pass the spectrophotometer. That enables a complete closed loop color control of a printer.
In the specific disclosed embodiment below the respective (different output color) LED illuminators of the spectrophotometer are normally sequentially illuminating different color test patches on test sheets in the color printer output path as they pass the spectrophotometer, for detection by a photosensor of the reflections from the respective color test patches of those respective color illuminations. In this exemplary automatic re-calibration system embodiment there is a white tile test standard surface mounted opposite to the spectrophotometer position in the printer output path, i.e., on the other side of the printer output path. These same LED""s of the spectrophotometer are sequentially activated in selected inter-sheet gaps (the spaces and times in between printed sheets), and the resultant spectrophotometer photosensor signal output for each activated LED is compared to stored values to provide calibration data for the spectrophotometer. This system allows for frequent automatic re-calibration without having to remove the spectrophotometer from the printer or perform any other manual operations with either the spectrophotometer or any test tiles or other white or color reflectance test surfaces.
However, color measurements, and/or the use of color measurements for various quality or consistency control functions, are also important for many other different technologies and applications, such as in the production of textiles, wallpaper, plastics, paint, inks, etc. This, the disclosed system may have applications in some of those other fields for on-line color testing where these materials or objects to be color tested and/or interspersed test sheets are also moving as sheets in a defined path with spaces therebetween. Thus, although the specific exemplary embodiment herein is of a preferred automatic re-calibration system for an on-line color printer color spectrophotometer, it will be appreciated that this exemplary re-calibration system is not limited to this specific exemplary spectrophotometer or its application as disclosed in this example.
By way of background, studies have demonstrated that humans are particularly sensitive to spatial color variations. Typical full color printing controls, as well as typical color controls in other commercial industries, still typically utilize manual off-line color testing and frequent manual color adjustments by skilled operators. Both the cost and the difficulty of on-line use of prior color measurement apparatus and control systems, and the need for manual re-calibration steps, has heretofore inhibited automation of many of such various commercial color testing and color adjustment systems. The disclosed system addresses both of those concerns.
As used in the patent claims and elsewhere herein unless otherwise specifically indicated, the term xe2x80x9cspectrophotometerxe2x80x9d may encompass a spectrophotometer, calorimeter, and densitometer, as broadly defined herein. That is, the word xe2x80x9cspectrophotometerxe2x80x9d is to be given the broadest possible definition and coverage in the claims herein, consistent with the rest of the claims themselves. The definitions or uses of terms vary or differ among various scientists and engineers. However, the following is an attempt to provide some simplified clarifications relating and distinguishing the respective terms xe2x80x9cspectrophotometerxe2x80x9d, xe2x80x9ccalorimeterxe2x80x9d, and xe2x80x9cdensitometerxe2x80x9d, as they may be used in the specific context of specification examples of providing components for an on-line color printer color correction system, but not as limitations.
A typical xe2x80x9cspectrophotometerxe2x80x9d measures the reflectance of an illuminated object of interest over many light wavelengths. Typical prior spectrophotometers in this context use 16 or 32 channels measuring from 400 nm to 700 nm or so, to cover the humanly visible color spectra or wavelength range. A typical spectrophotometer gives color information in terms of measured reflectances or transmittances of light, at the different wavelengths of light, from the test surface. (This is to measure more closely to what the human eye would see as a combined image of a broad white light spectra image reflectance, but the spectrophotometer desirably provides distinct electrical signals corresponding to the different levels of reflected light from the respective different illumination wavelength ranges or channels.)
A xe2x80x9ccolorimeterxe2x80x9d normally has three illumination channels, red, green and blue. That is, generally, a xe2x80x9ccalorimeterxe2x80x9d provides its three (red, green and blue or xe2x80x9cRGBxe2x80x9d) values as read by a light sensor or detector receiving reflected light from a color test surface sequentially illuminated with red, green and blue illuminators, such as three different color LED""s or three lamps with three different color filters. It may thus be considered different from, or a limited special case of, a xe2x80x9cspectrophotometerxe2x80x9d, in that it provides output color information in the trichometric quantity known as RGB.
Trichometric quantities may be used for representing color in three coordinate space through some type of transformation. Other RGB conversions to xe2x80x9cdevice independent color spacexe2x80x9d (i.e., RGB converted to conventional L*a*b*) typically use a color conversion xe2x80x9clookup tablexe2x80x9d system in a known manner. (Examples are provided in patents cited below, and elsewhere.)
A xe2x80x9cdensitometerxe2x80x9d typically has only a single channel, and simply measures the amplitude of light reflectivity over a range of wavelengths, which may be wide or narrow. The output of the densitometer detector is programmed to give the optical density of the sample. A densitometer is basically xe2x80x9ccolor blindxe2x80x9d. For example, a cyan patch and magenta patch could have the same optical densities as seen by a densitometer, but, of course, are different colors.
A multiple LED""s reflectance spectrophotometer, as in the example of the embodiment herein, may be considered to belong to a special case of spectrophotometers. (Others, with different respective illumination sources, can be flashed Xenon lamp spectrophotometers, or QH spectrophotometers.) It is a spectrophotometer programmed to give truer reflectance values by using more than 3 channel measurements (e.g., 10 or more channel measurements), with conversion algorithms. That is in contrast to normal calorimeters, which cannot give true, human eye related, reflectance spectra measurements, because they have insufficient measurements for that (only 3 measurements).
As noted, the type of spectrophotometer in the disclosed embodiment is a spectrophotometer especially suitable for being mounted in the printed sheets output path of a color printer to optically evaluate the output sheets as they move past the spectrophotometer. In particular, to measure a limited number of color test patch samples printed by the printer on actual printed sheet output of the printer during regular or selected printer operation intervals (between normal printing runs or print jobs). These color test sheet printing intervals may be at regular timed intervals, and/or at each machine xe2x80x9ccycle-upxe2x80x9d, or as otherwise directed by the system software.
As is additionally disclosed in the embodiment herein, it is advantageous to provide dual-mode color test sheets/banner sheets, in which multiple color patches of different colors are printed on otherwise blank areas of each, or selected, banner, cover, or other inter-document or print job separator sheets. Different sets of colors may be printed on different banner sheets. Providing this dual use of such sheets saves both print paper and printer utilization time, and also provides frequent color re-calibration opportunities, where the printing system is one in which banner sheets are being printed at frequent intervals anyway. It is quite common for shared user printers, even those with mailbox system job separators, to automatically generate and print a banner sheet immediately preceding the first page of each actual document being printed. A banner sheet may typically have automatically printed thereon by system software a limited amount of printed information about that particular document or print job, such as the print job or document name, user name, printer name, host system name, file name, date, numbers of pages, etc. Some examples of banner sheets are disclosed in Xerox Corp. U.S. Pat. Nos. 5,547,178 and 5,316,279.
That is, another disclosed feature herein is a system of dual mode sheets which can provide the combined functions of banner sheets and color test sheets on the same sheet. Document xe2x80x9cbannerxe2x80x9d sheets are already being generated and printed in many printers. As disclosed herein, the same banner sheets may now also used to variously print thereon the multiple color test patches for the spectrophotometer analysis by the disclosed output color control system. This dual mode sheet usage system saves substantial amounts of otherwise wasted paper otherwise being used for non-imaged color test sheets. Furthermore, it enables frequent color re-correction inputs with no reduction in printer productivity. That is, normal document printing in a color printer does not have to be relatively frequently interrupted to print extra (non-document imaged) color test sheets to keep each color printer re-calibrated. As noted elsewhere herein, relatively frequent color re-calibration of a color printer is desirable, since the colors actually printed on the output can change or drift out of calibration with the intended colors for various known reasons. For example, changes in the selected or loaded print media (differences paper or plastic sheet types, materials, weights, calendering, coating, humidity, etc.), changes in the printer""s ambient conditions, changes in image developer materials, aging or wear of printer components, varying interactions of the different colors being printed, etc.
This dual mode system can be provided without hardware changes or costs with a combination of: (a) existing (or minor modifications of the) software for feeding, generating and printing the banner sheets, such as condensing and/or moving the location of the banner information printed on the banner sheets, with (b) existing (or minor modifications of the) software for generating and printing color test sheets with multiple test patches of different colors.
Banner sheets are normally printed at more than frequent enough intervals to provide for very frequent automatic re-calibration test sheets to frequently update the color printing electronic image information and/or color printing sub-systems. Thus, not every banner sheet needs to be a dual mode sheet with color test patches. In fact, for the normal situation of relatively few document or print job pages per banner sheet in a shared user printer environment, it may be desirable to reduce color toner usage by only using those banner sheets as color test sheets which occur at more than a preset time interval apart. It may also be desirable to only provide color test patterns on those banner sheets which are the banner sheets for a document which is being color printed, not the banner sheets for black and white documents, except perhaps at cycle-up or after a long time delay.
An additional feature which can be provided with this system is to tailor or set the particular colors of the test patches on a particular banner sheet to those colors which are about to be printed on the specific document for that banner sheet, i.e., the document pages which are printed immediately subsequent to that banner sheet (the print job identified by that banner sheet). This can provide a xe2x80x9creal timexe2x80x9d color correction for the color printer which is tailored to correct printing of the colors of the next document to be printed.
The preferred implementations of the systems and features disclosed herein may vary depending on the situation. Also, various of the disclosed features or components may be alternatively used for such functions as gray scale balancing with gray test patches, turning on more than one of the illumination sources at once, such as oppositely positioned LED""s, etc.
However, in using dual mode color test banner sheets, or other color test sheets, in the specifically disclosed on-line printer system, it will be appreciated that the color test patches on the sheets should be compatible with the performance metrics of the spectrophotometer or other color sensor being used, and the color test patches are printed on the sheet in locations within the sensor""s field of view as the sheet is fed past the sensor""s field of view.
It will also be appreciated that these test patch images and colors may be automatically sent to the printer imager from a stored data file specifically designed for printing the dual mode banner sheet or other color test sheet page, and/or they may be embedded inside the customer job containing the banner page. That is, the latter may be directly electronically associated with the electronic document to be printed, and/or generated or transmitted by the document author or sender.
After the spectrophotometer or other color sensor reads the colors of the test patches, the measured colors may be processed inside the system controller or the printer controller to produce or modify the tone reproduction curve. The color test patches on the next banner page, and the customer document pages of the next print job may then be printed with that new tone reproduction curve. This process may be continuously repeated for each subsequent print job and its banner page so as to generate new or further corrected tone reproduction curves from each subsequent banner page. If the printer""s color image printing components and materials are relatively stable, with only relatively long term drift, the tone reproduction curve produced by measuring colors off the normal single banner page for each print job, and using this closed loop control system, will be the right curve for achieving consistent colors for at least one or even a substantial number of customer print jobs printed thereafter.
However, if there are substantial changes in the print media being used by the printer, or other sudden and major disturbances in the printed colors (which can be detected by the spectrophotometer output in response to the test patches on the next dual mode banner sheet or other color test sheet) then the subsequent customer print job may have incorrect color reproduction. In these situations of customer print media changes in the printer (or new print jobs or job tickets that specify a change in print media for that print job), where that print media change is such that it may substantially affect the accuracy of the printed colors for that subsequent print job, it is not desirable to continue printing and then have to discard the next subsequent print jobs printed with customer-unacceptable colors. In that situation it is preferable to interrupt the normal printing sequence once the sudden color printing disturbance is detected and to instead print plural additional color test sheets in immediate succession, with different color test patch colors, to sense and converge on a new tone reproduction curve that will achieve consistent color printing for that new print media, and only then to resume the normal printing sequence of customer print jobs. Thus, the subsequent customer print jobs would then use the final, restabilized, tone reproduction curve obtained after such a predetermined number of sequential plural color test sheets or dual mode banner pages have been printed.
However, this patent application is not related to or limited to any particular one of the various possible (see, e.g., various of the cited references) algorithms or mathematical techniques for processing the electronic signals from the spectrophotometer to generate color correction tables, tone reproduction curves or other color controls, and hence those need not be further discussed herein.
As noted, the disclosed re-calibration system embodiment is an important feature for a practical on-line xe2x80x9creal timexe2x80x9d color printing color calibration or correction system which regularly measures the actual colors currently being printed on the printed sheets being outputted by the printer, as compared to the intended (or selected, or xe2x80x9ctruexe2x80x9d) colors of the electronic document images being inputted to the printer for printing.
A low cost and relatively simple, yet easily re-calibrated spectrophotometer, as disclosed in the example below, is highly desirable for such a xe2x80x9ccolorimetryxe2x80x9d function for such an on-line color correction system, since a dedicated spectrophotometer must be provided for each printer. A patent of particular background interest as to using a type of spectrophotometer at the printed sheets output of a color printer is Xerox Corp. U.S. Pat. No. 5,748,221 issued May 5, 1998 to Vittorio Castelli, et al, filed Nov. 1, 1995 (D/95398).
Further by way of background, various possible color correction systems can employ the output signals of spectrophotometers, using various sophisticated feedback, correction and calibration systems, which need not be discussed in any further detail here, since the general concepts and many specific embodiments are disclosed in many other patents (including those cited hereinbelow) and publications. That is, to electronically analyze and utilize the spectrophotometer or other electronic printed color output information with a feedback analysis system for the color control systems for the printer. It is desirable in such systems to be able to use a reduced (smaller) number of color patch samples, printed at intervals during the regular printing operation of the printer, yet still provide relatively substantially continuous updating correction of the printer""s color renditions over a wide or substantially complete color spectra. Noted especially is Xerox Corp. filed Jan. 21, 1997, by Steven J. Harrington as U.S. application Ser. No. 08/786,010, now issued as U.S. Pat. No. 6,178,007 on Jan 23, 2001, published by the European Patent Office on Jul. 22, 1998, as EPO publication No. 0 854 638 A2; and Apple Computer, Inc. U.S. Pat. No. 5,612,902, issued Mar. 18, 1997, to Michael Stokes.
Another example of a test sheet with color test patches automatically generated by a color printer, for operator use, is shown in Xerox Corp. U.S. Pat. No. 5,604,567 issued Feb. 18, 1997 to Peter H. Dundas, et al.
Color correction and/or color control systems should not be confused with color registration systems or sensors. Those systems are for insuring that colors are correctly printed accurately superposed and/or accurately adjacent to one another, such as by providing positional information for shifting the position of respective color images being printed.
Other background patents which have been cited as to color control or correction systems for printers include Xerox Corp. U.S. Pat. 5,963,244 issued Oct. 5, 1999 to L. K. Mestha, et al entitled xe2x80x9cOptimal Reconstruction of Tone Reproduction Curvexe2x80x9d (using a lookup table and densitometer readings of photoreceptor sample color test patches to control various color printer parameters); and U.S. Pat. No. 5,581,376, issued December 1996 to Harrington; U.S. Pat. No. 5,528,386 issued Jun. 18, 1996 to Rolleston et al.; U.S. Pat. No. 4,275,413 issued Jun. 23, 1981 to Sakamoto et al.; U.S. Pat. No. 4,500,919 issued Feb. 19, 1985 to Schreiber; U.S. Pat. No. 5,416,613 issued May 16, 1995 to Rolleston et al.; U.S. Pat. No. 5,508,826 issued Apr. 16, 1996 to Lloyd et al.; U.S. Pat. No. 5,471,324 issued Nov. 28, 1995 to Rolleston; U.S. Pat. No. 5,491,568 issued Feb. 13, 1996 to Wan; U.S. Pat. No. 5,539,522 issued Jul. 23, 1996 to Yoshida; U.S. Pat. No. 5,483,360 issued Jan. 9, 1996 to Rolleston et al.; U.S. Pat. No. 5,594,557 issued January 1997 to Rolleston et al.; U.S. Pat. No. 2,790,844 issued April 1957 to Neugebauer; U.S. Pat. No. 4,500,919 issued February 1985 to Schreiber; U.S. Pat. No. 5,491,568 issued Feb. 13, 1996 to Wan; U.S. Pat. No. 5,481,380 to Bestmann issued Jan. 2, 1996; U.S. Pat. No. 5,664,072 issued Sep. 2, 1997 to Ueda et al.; and U.S. Pat. No. 5,544,258 issued Aug. 6, 1996 to Levien.
By way of further background on the subject of technology for automatic color correction for color printers or other reproduction apparatus, especially such systems utilizing feedback signals from a calorimeter or spectrophotometer (as noted, those terms may be used interchangeably herein), and/or automatically measuring the actually printed colors of test patches on printed copy sheets as they are being fed through the output path the printer, there is noted the following: the above-cited Xerox Corporation U.S. Pat. No. 5,748,221 filed Nov. 1, 1995 and issued May 5, 1998 to V. Castelli, et al, entitled xe2x80x9cApparatus for Colorimetry, Gloss and Registration Feedback in a Color Printing Machinexe2x80x9d, (noting especially the output path test print colorimeter detector details); the above-cited Apple Computer, Inc. U.S. Pat. No. 5,612,902, issued Mar. 18, 1997 to Michael Stokes; Xerox Corporation U.S. Pat. No. 5,510,896 issued Apr. 23, 1996 to Walter Wafler, filed Jun. 18, 1993 (see especially Col. 8 re color calibration from information from a scanned color test copy sheet as compared to original color image information); and Xerox Corporation U.S. Pat. No. 5,884,118 issued Mar. 16, 1999 to Mantell and L. K. Mestha, et al, entitled xe2x80x9cPrinter Having Print Output Linked to Scanner Input for Automated Image Quality Adjustmentxe2x80x9d (note especially Col. 6 lines 45-49).
U.S. Patents of interest to color correction in general, but which may be useful with, or provide background information for, the above or other systems, includes the above-cited Xerox Corporation U.S. Pat. No. 5,594,557, filed Oct. 3, 1994 and issued Jan. 14, 1997 to R. J. Rolleston et al., entitled xe2x80x9cColor Printer Calibration Correcting for Local Printer Non-Linearitiesxe2x80x9d; Seiko Epson Corp. U.S. Pat. No. 5,809,213, provisionally filed Feb. 23, 1996 and issued Sep. 15, 1998 to A. K. Bhattacharjya re reduced color measurement samples; and Splash Technology, Inc. U.S. Pat. No. 5,760,913 filed Feb. 12, 1996 and issued Jun. 2, 1998 to Richard A. Falk in which a calibration image is scanned using a scanner coupled to the printing system with a personal computer.
Also noted are pending Xerox Corp. U.S. application Ser. No. 09/083,202 filed May 22, 1998 by Mark A. Scheuer, et al., entitled xe2x80x9cDevice Independent Color Controller and Methodxe2x80x9d, Ser. No. 09/083,203, filed May 22, 1998 by Lingappa K. Mestha, entitled xe2x80x9cDynamic Device Independent Imagexe2x80x9d, now issued as U.S. Pat. No. 6,157,469 on Dec. 5, 2000, Ser. No. 09/232,465, filed Jan. 19, 1999 by Martin E. Banton, et al., entitled xe2x80x9cApparatus and Method for Using Feedback and Feedforward in the Generation of Presentation Images In A Distributed Digital Image Processing Systemxe2x80x9d, and Ser. No. 09/221,996, filed Dec. 9, 1998 by Lingappa K. Mestha, et al., entitled xe2x80x9cColor Adjustment Apparatus and Methodxe2x80x9d.
As further well-known background for the reader on the subject of difficulties in color correction of printers in general, computers and other electronic equipment generating and inputting color images or documents typically generate three-dimensional or RGB (red, green, blue) color signals. Many printers, however, can receive four-dimensional or CMYK (cyan, magenta, yellow, and black) signals as input, and/or can print with four such print colors (although the printed images can be measured as corresponding RGB values). A look-up table is commonly provided to convert each digital RGB color signal value to a corresponding digital CMYK value before or after being received by the printer. Another difficulty is that a theoretical printer which had ideal toner, ink or dye printing materials colors and printing behavior would have a one-to-one correspondence of cyan-to-red, magenta-to-green, and yellow-to-blue. This would mean that when printed, the cyan ink would only absorb red light, the magenta ink would only absorb green light, and the yellow ink would only absorb blue light. However, real-world printers inherently have non-ideal printing materials colors and behaviors, and therefore have complex non-linear calorimetric responses. Also, interactions between the cyan, magenta, and yellow imaging materials exist, especially on the printed output, which result in unwanted or unintended absorptions of colors. Even after a printer is initially calibrated, such that one or a range of input digital CMYK values produce the proper color(s), the full spectrum of CMYK values and printed colors is not accurate. In other words, the colors asked or directed to be printed are not the same as the actual colors printed.
This discrepancy arises in part because the relationship between the digital input values that drive the printer and the resulting colorimetric response is a complex non-linear function. Labeling the response, or other values, as xe2x80x9ccolorimetricxe2x80x9d can indicate that the response or value has been measured by such an instrument. Adequately modeling the colorimetric response of a printer to achieve linearity across the entire available spectrum requires many parameters. Typically, a color correction look-up table is built which approximates the mapping between RGB calorimetric space and CMYK values, as taught in various of the above-cited references. Each RGB coordinate may be typically represented by an 8-bit red value, an 8-bit green value, and an 8-bit blue value. Although those RGB coordinates are capable of addressing a look-up table having 2563 locations, measuring and storing 2563 values is expensive. The look-up table is thus typically partitioned into a smaller size such as 16xc3x9716xc3x9716 (4096) table locations, each of which stores a four-dimensional CMYK value. Other CMYK values may then be found by interpolating the known CMYK values using an interpolation process, for example, trilinear or tetrahedral interpolation.
The color correction look-up table may be built by sending a set of CMYK digital values to the printer, measuring the calorimetric RGB values of the resulting color patches outputted by the printer, and generating the look-up table from the difference between the inputted values and the measured outputted values. More specifically, the color correction look-up table corrects for non-linearities, printing parameter variations, and unwanted absorptions of inks, so that the printer will print the true corresponding color.
After the color correction table is generated, the actual printer response tends to drift over time. To correct for the drift, the system is adjusted or recalibrated periodically. Recalibrating the color correction table involves periodically printing and remeasuring a set of test color patches which are then compared to an original set of color patches by calibration software. Remeasuring, however, has heretofore more typically been performed by a scanner or other measuring device which is remote from the printer being recalibrated. In that case, an operator must manually reconfigure the scanner and calibration software to properly recognize and measure the test color patches. This assumes that the operator can properly identify the test color patches being tested in accordance with the original printer and its test pattern properties. Furthermore, once a color correction table is generated, it must be associated with the correct printer, otherwise, a different printer will be recalibrated with an incorrect correction table. The above-cited references on automatic, on-line, color correction note the important advantages of being able to provide direct output color measurements for each printer.
The present invention thus also provides for a new and improved method of assisting in the calibrating a color printer which overcomes various above-referenced and other problems. However, it will be appreciated that although the specific embodiment is described with particular reference to desirable applications for calibrating and regularly re-calibrating color printers and/or refining color correction tables, that what is disclosed herein will also find various applications in other printing devices and other color testing and correction systems.
As discussed, in high quality color reprographic applications, it is highly advantageous to monitor system calorimetric performance on-line through the use of an integrated spectrophotometer. That is, to have the printing device automatically fairly frequently generate calibration prints on otherwise normally printed sheets with color patches based on digital test pattern generations, and to have an on-line spectrophotometer in the printer output read those moving sheet printed color test patches accurately to provide printed output color measurement signals. This requires a spectrophotometer capable of effectively operating in that environment and under those conditions, which are not typical for conventional laboratory spectrophotometers.
Turning to details of this particular specific embodiment, traditional spectrophotometers normally require, for uniform output, that the target, including a calibration surface target, be precisely positioned with respect to the spectrophotometerxe2x80x94typically, by being held stationary, for nearly direct contact. In contrast, disclosed herein is a spectrophotometer that is relatively insensitive to the positioning of the object or target of interest. This spacing insensitivity enables this spectrophotometer to be positioned at any convenient location in the paper path of a printing machine, rather than at a location where the paper position is tightly controlled. It may even be fitted into the output sheet stacker tray of various existing color printers.
A specific feature of the specific embodiment disclosed herein is to provide in a color correction system for a color printer with an output path for printed color sheets, including printed test sheets with printed color test patches, in which a calibrated spectrophotometer is mounted in said printer output path for sensing the colors printed on a test patch on a test sheet as said test sheets are moving past said spectrophotometer in said printer output path, wherein said spectrophotometer includes a plurality of illumination sources for sequentially illuminating a said test patch with different illumination colors, a photodetector sensor providing electrical output signals, and a lens system with a field of view for transmitting said illumination from said test patch to said photodetector sensor, whereby said photodetector sensor provides different said electrical output signals in response to viewing said different illumination colors from said sequential illuminations of said test patch by said plural illumination sources, wherein said spectrophotometer is mounted at one side of said output path of said color printer for measuring the colors of said test patches with said spectrophotometer, the improvement in an automatic re-calibration system for calibrating said spectrophotometer, comprising: a stationary calibration test patch mounted adjacent to the opposite side of said output path of said color printer from said spectrophotometer within said field of view of said lens system and positioned to be sequentially illuminated by said plural illumination sources to provide respective calibration signals from said photodetector sensor when a said printed color sheet is not in said output path in between said spectrophotometer and said calibration test patch.
Further specific features disclosed herein, individually or in combination, include those wherein said calibration test patch is a standard white tile test surface; and/or wherein at least a portion of said output path of said color printer for printed color sheets adjacent said calibration test patch is defined by spaced apart baffles; and/or wherein a sheet path baffle defines at least one side of said output path adjacent said calibration test patch, and said sheet path baffle has an aperture therein opposite from said spectrophotometer, and wherein said calibration test patch is mounted outside of said output path behind said aperture but within said field of view of said lens system through said aperture; and/or wherein said plural illumination sources comprise multiple differently filtered illumination sources with circuitry for rapidly sequentially individually illuminating said calibration test patch with respective different illumination colors, and wherein said multiple illumination sources are mounted arrayed around said photodetector sensor and said lens system therefor in said spectrophotometer, and wherein each said illumination source has an individual lens system for angularly illuminating said calibration test patch at substantially the same angle; and/or wherein said multiple illumination sources are provided by multiple LED""s, each a with different color filter; and/or wherein said multiple illumination sources comprise 10 or more LED""s; and/or wherein said multiple illumination sources are in a circular pattern surrounding said photodetector sensor and defining a central axis, and wherein said photodetector sensor and said lens system for transmitting said illumination from said test patch to said photodetector sensor are aligned with said central axis; and/or wherein said color printer has a conventional control system for tracking said printed sheet positions in said output path, and wherein said printer control system provides control signals for periodically actuating said plural illumination sources when said printed sheets are not within said field of view of said lens system to illuminate said calibration test patch; and/or a method of automatically re-calibrating a spectrophotometer mounted at one side of the printed sheets output path of a color printer for measuring the colors printed on colored test patches on test sheets as said-test sheets are moved past said spectrophotometer in said printer output path and said test patches are sequentially illuminated with a plurality of different illumination colors by a plurality of illumination sources, which different illumination colors are reflected from said test patches and detected by a photodetector sensor providing electrical output signals, comprising: sequentially illuminating with said same plurality of illumination sources a stationary calibration test patch which is mounted oppositely from said spectrophotometer on the opposite side of said output path of said color printer to provide respective calibration signals from said electrical output signals of said photodetector sensor at intervals when a said printed color sheet is not in said output path in between said spectrophotometer and said calibration test patch; and/or wherein said calibration test patch is a standard white tile test surface; and/or further comprising automatically tracking the positions of said printed sheets in said output path and sequentially actuating said same plural illumination sources at selected times when said printed sheets are not in between said spectrophotometer and said calibration test patch.
The disclosed system may be connected, operated and controlled by appropriate operation of conventional control systems. It is well known and preferable to program and execute various control functions and logic with software instructions for conventional or general purpose microprocessors, as taught by numerous prior patents and commercial products. Such programming or software may of course vary depending on the particular functions, software type, and microprocessor or other computer system utilized, but will be available to, or readily programmable without undue experimentation from, functional descriptions, such as those provided herein, and/or prior knowledge of functions which are conventional, together with general knowledge in the software and computer arts. Alternatively, the disclosed control system or method may be implemented partially or fully in hardware, using standard logic circuits or single chip VLSI designs.
In the description herein the term xe2x80x9csheetxe2x80x9d refers to a usually flimsy physical sheet of paper, plastic, or other suitable physical substrate for images, whether precut or web fed. A xe2x80x9ccopy sheetxe2x80x9d may be abbreviated as a xe2x80x9ccopyxe2x80x9d, or called a xe2x80x9chardcopyxe2x80x9d As will be noted, printed sheets may be referred to as xe2x80x9coutputxe2x80x9d. A xe2x80x9cprint jobxe2x80x9d is normally a set of related printed sheets, usually one or more collated copy sets copied from a one or more original document sheets or electronic document page images, from a particular user, or otherwise related.
As to specific components of the subject apparatus, or alternatives therefor, it will be appreciated that, as is normally the case, some such components are known per se in other apparatus or applications which may be additionally or alternatively used herein, including those from art cited herein. All references cited in this specification, and their references, are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features, and/or technical background. What is well known to those skilled in the art need not be described here.